Nearly-free electron gap at a zone boundary
For a real periodic one-dimensional potential of period a, define V_G=a⁻¹∫₀ᵃV(x)e^(−iGx)dx, G=2π/a, and ε_k=ħ²k²/(2m).
Key knowledge
For a real periodic one-dimensional potential of period a, define V_G=a⁻¹∫₀ᵃV(x)e^(−iGx)dx, G=2π/a, and ε_k=ħ²k²/(2m). Near k=G/2, the coupled plane waves |k⟩ and |k−G⟩ have the leading two-state Hamiltonian with diagonal ε_k+V₀, ε_(k−G)+V₀ and off-diagonal V_G,V_G*. Its energies are V₀+(ε_k+ε_(k−G))/2 ± √[((ε_k−ε_(k−G))/2)²+|V_G|²]. At the boundary the leading gap is 2|V_G|.
Values and equations
H_eff=[[ε_k+V₀,V_G],[V_G*,ε_(k−G)+V₀]]; E_±=V₀+(ε_k+ε_(k−G))/2±√[((ε_k−ε_(k−G))/2)²+|V_G|²]; ΔE_at_boundary=2|V_G|
- H_eff,V₀,V_G,ε_k,ε_(k−G),E_±,ΔE
- Two-state Hamiltonian, Fourier coefficients and energies J
- a,x
- Lattice period and position m
- G,k
- Reciprocal and electron wave numbers m⁻¹
- m
- Effective model particle mass kg
- ħ
- Reduced Planck constant J s
Conditions and scope
- Nonrelativistic effective single-particle model, m>0 and a>0, static real potential and normalized plane waves; Fourier convention V(x)=Σ_n V_(nG)e^(inGx) gives V_(−G)=V_G*. The uniform V₀ only shifts the energy zero.
- The two nearly degenerate states are mixed exactly within this subspace; couplings to the other plane-wave levels must be small relative to their separations for neglected higher-order corrections to be controlled. A vanishing V_G means no first-order gap from this coupling, not a proof of no higher-order gap or of a higher-dimensional global band gap.
Sources · 1
References
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Evidence and review
Solid State Physics: third-year lecture notes (2012): Sections14.1–14.1.1, equations14.1–14.8 and exact-zone-boundary eigenvalues: potential Fourier matrix elements, two-state secular equation and 2|V_G| splitting. The record uses the equivalent basis (k,k−G), restores the constant V₀ and obtains the near-boundary roots by solving the displayed quadratic; no higher-dimensional global-gap claim is inferred.
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